#include <iostream>

int main() {
    double a = 0;
    int b = 0;
    std::cout << "please input" << std::endl;
    std::cin >> a;
    b = a * 10;
    if (a == 0) {
        std::cout << 1 << std::endl;
    } else {
        if (b < 0 || b % 10 != 0) {
            std::cout << "invalid input" << std::endl;
        } else {
            b = a;
            while ((a - 1) > 0) {
                b = b * (a - 1);
                a--;
            }
            std::cout << b << std::endl;
        }
    }
}